Olefin, one of the major source materials in the chemical industry, is mainly produced by cracking naphtha or natural gas. During this process, paraffinic hydrocarbons and acetylenic compounds having similar boiling points are produced together. Thus, a complicated separation and purification process is required to obtain pure olefins. In particular, the acetylenic compounds act as a catalytic poison in the polyolefin production process, and degrade the quality of the product, and is subject to explode or block the fluid flow when converted to solid polyacetylenic compounds and accumulated during the production process. Therefore, the removal of the acetylene compounds is essential.
For practical applications, traces of acetylenic compounds included in olefin are converted into olefins via hydrogenation in the presence of a catalyst. However, olefin produced during the hydrogenation of acetylenic compounds or olefin used as a reagent can react together and form paraffins; this might cause a loss of olefin.
For this reason, a catalyst capable of selectively hydrogenating acetylenes has to be used to remove the acetylenic compounds. Currently, palladium supported on α-alumina is the most frequently used and commercially available catalyst. However severe catalyst poisoning due to excessive paraffin production from high hydrogenation activity and carbon deposition requires an additional regeneration process of the catalyst.
In addition to hydrogenation, low-temperature distillation, liquid absorption, solid adsorption, membrane separation, or the like are known as methods for removing the acetylenic compounds. Among them, low-temperature distillation and liquid absorption are frequently employed to separate unsaturated compounds such as carbon monoxide (CO) or olefin from gaseous mixtures. However, low-temperature distillation requires expensive equipments, and high operation costs. The liquid absorption method using volatile organic solvents such as dimethylformamide (DMF) or N-methylpyrrolidone (NMP) requires an additional separation/purification process of removing olefins from acetylenic compounds dissolved in an absorbent to obtain pure acetylenic compounds because of low selectivity. In addition, loss of the volatile organic solvents during repeated regeneration of the absorbent is economically unfavorable.
U.S. Pat. Nos. 4,019,879 and 4,034,065 disclose methods of removing unsaturated compounds such as CO via adsorption using molecular sieves. However, their adsorption capacity is limited and high temperature and high vacuum is required for degassing. U.S. Pat. No. 4,717,398 discloses a method of removing unsaturated compounds by a pressure swing process using an adsorbent obtained using copper [Cu(I)]-exchanged faujasite zeolite.
German Patent No. 2,059,794 discloses a method of removing unsaturated compounds including acetylene using a liquid absorbent containing a Cu(I) compound and an alkanolamine such as monoethanolamine as main components. However, it requires an additional purification apparatus because of contamination of the final product by the alkanolamine and co-adsorption of olefin. Ind. Eng. Chem. Res. 2571(1998) discloses a method of separating unsaturated compounds from paraffins using a Cu(I) or Ag(I) compound solution reacting reversibly with olefin and acetylene. However, it requires a complicated regeneration process because of low stability of the adsorbent.
U.S. Pat. No. 3,758,603 discloses a method of separating unsaturated compounds from saturated compounds using a liquid barrier prepared by supporting silver salt on a porous separation membrane. The liquid barrier technique is disadvantageous as silver ions are lost by supplied gases and the solvent evaporate easily. As a result, the separation efficiency cannot maintain for a long time. Even when a cation exchange membrane is used to prevent the silver ion loss as described in U.S. Pat. No. 4,318,714, a water content in the separation membrane has to be maintained above a certain level as in the case where an immobilized liquid barrier is used, because the facilitated transport occurs only in the presence of water, and water has to be removed later after the separation. Further, since the separation membrane has to be thick with a thickness of 100 to 500 μm or larger, it is impractical. In addition, the separation efficiency is not satisfactory.
Although the aforesaid methods using the Cu(I) or Ag(I) compound are applicable to the separation of unsaturated hydrocarbons from saturated hydrocarbons, they are inapplicable to the separation of a mixture of unsaturated compounds. It is because the separation selectivity is fairly low since the Cu(I) or Ag(I) compound forms n-complexes having bond strengths comparable to those of double or triple bonds.